Typifier skeleton

[kvark]

There is a small TODO in the current WGSL parser:

                Token::Separator('.') => {
                    let _ = lexer.next();
                    let _name = lexer.next_ident()?;
                    let expr = crate::Expression::AccessIndex {
                        base: handle,
                        index: 0, //TODO: compute from `name`
                    };
                    handle = ctx.expressions.append(expr);
                }

We have a base expression, and we need to access a sub-field. The IR doesn't care about names, so in case of a structure it just needs an index into the member list. The problem, however, is realizing that the base type is a structure. In order to compute them, we essentially need to know all the types of all the expressions.

This is where Typifier comes in. It's a generic processor of IR that helps to build that type information for a growing list of expressions.

[kvark]

Partially reviewed by @jrmuizel. Merging.